1. Field of the Invention
The present invention relates to an image enlargement technique.
2. Description of the Related Art
Unsharp masking is a process of emphasizing the edge portion of an image. This designation derives from the presence of image data smoothing included in a process of emphasizing the edge.
In unsharp masking, smoothing is executed for original image data Ia as a process target to generate a smoothed image Ib. Smoothing is a process of averaging a pixel of interest and neighboring pixels around it by using a smoothing filter 201 as shown in FIG. 2A and changing the value of the pixel of interest, thereby shading the image. Image data Is with improved sharpness is generated based on the smoothed image data Ib and original image data Ia. These images hold a relationship given byIs=Ia+α(Ia−Ib)  (1)
Equation (1) will be described briefly. An edge component (i.e., a high-frequency component of an image) is extracted by calculating the difference between the original image Ia and the smoothed image Ib. The edge component is emphasized by multiplying it by an emphasis coefficient α. Then, the emphasized edge component is added to the original image Ia, thereby obtaining the image data Is with improved sharpness.
The print resolutions of printing apparatuses are steadily rising recently. A print resolution indicates the number of pixels per unit length (or unit area). To print by using a printing apparatus having a high print resolution, the number of pixels of image data also needs to increase. That is, a print target image needs to be large, the resultant image data Is of equation (1) is generally enlarged as needed.
However, if the image data that has undergone unsharp masking is enlarged, the specially emphasized edge portion smoothes, resulting in little effect of unsharp masking.
A technique is known which executes unsharp masking by setting the emphasis coefficient α of equation (1) in accordance with the image enlargement ratio and then enlarges the image (e.g., Japanese Patent Laid-Open No. 2003-141532). This reference also discloses another method which enlarges an image first and then executes edge emphasis by using the emphasis coefficient α with a large value.
In the above-described technique, if an image is enlarged after edge emphasis, the edge smoothes, resulting in noticeable image degradation. Even when edge emphasis is done after enlargement, the smoothing effect deteriorates because the edge area of the enlarged image is large. For these reasons, it is difficult to emphasize the edge portion only by making the emphasis coefficient α large. Conversely, the process uselessly emphasizes parts other than the edge portion, and noise in the image increases.
To cope with an increase in edge area, a method of increasing the size of a filter for edge detection of an enlarged image is known (e.g., Japanese Patent Laid-Open No. 2003-274157), although this technique is not related to unsharp masking. According to this reference, a smoothing filter with a larger size can cope with an increase in edge area. However, parts other than the edge area are also emphasized, and noise in the image increases because of the enhanced smoothing effect.